The same subject doesn't determine the thread perfectly, you really need to also
include a header along the lines of
References: <wallis1000000$194.82.45.24$.002801c60234$caf13c40$a202020a@oem9>
which is based on the
Message-ID: <9e4313e10512160345q2dd6e27exac27af963b64c6ea@mail.gmail.com>
header from the email you're replying to (and possibly, other references, I
don't remember exactly).

In answer to the original question, it's non-trivial, since digest mode is a
summarisation, it probably doesn't include the appropriate headers that you'd
need to copy bits out of in order to attach to the thread.
Using the same subject will get people's attention tho, it just won't be obvious
which post you're replying to, since it won't appear to be threaded in all email
clients.

Andrew

>
> lImbus
>
> On 12/16/05, alex wallis <wallis@andrelouis.com> wrote:
> > hi list.
> > I have my list subscription set to digest mode so how can i reply to an
> > existing thread without starting a knew message with the same subject line
> > so its part of the thread?
>
>

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This email was sent from Netspace Webmail: http://www.netspace.net.auReceived on Fri Dec 16 22:26:31 2005